The invention relates to a circuit arrangement for producing at least an output direct voltage from an at least significantly sinusoidal a.c. voltage.
Circuit arrangements of this type are basically known from the monograph "Schaltnetzteile in der Praxis" by Otmar Kilgenstein. In this treatise a sinusoidal mains a.c. voltage is first converted, with the aid of a bridge rectifier and a storage capacitor into a direct voltage, which is then used as the input voltage for a switched-mode power supply which produces one or a plurality of output direct voltages. These voltages can be electrically insulated from or d.c. coupled to the mains a.c. voltage. A considerable disadvantage of this prior art is the fact that electro-magnetic interferences are formed, both in the range of switching frequencies and their harmonics, i.e. consequently at relatively high frequencies, and also in the range of the mains frequency, i.e. in the low frequency range. In the latter case a significant distortion of the currents taken from the mains becomes noticeable in a very disturbing manner.
A further problem to be considered in switched-mode power supplies of the above prior art is that switching losses occur at high-frequency switches of the switched-mode power supply. These switching losses which increase versus the increase in the switching frequency, result in an impermissible, thermal load of more specifically the high-frequency switch and are an obstacle to an increase of the switching frequency to above a limit set by this thermal load. An increase of the switching frequency is on the other hand very desirable, for example for reducing the magnetic components of the switched-mode power supply.
From the monograph "Schaltnetzteile" by W. Hirschmann (Siemens) it is known, for the purpose of reducing distortions of the currents taken from mains, i.e. for reducing the harmonics of the mains frequency in the curent taken from mains, to connect the switched-mode power supply directly to the bridge rectifier, which is fed from mains, without the intermediacy of a filter capacitor. By controlling in a corresponding manner, the turn-on and turn-off moments, respectively, of the high-frequency clocked switch in the switched-mode power supply a current which is at least to a significant extent free from harmonics and varies sinusoidally versus the mains frequency can be taken from mains. This circuit has however the disadvantage that the power absorbed by it has in addition to a constant pan also a part which fluctuates at twice the mains frequency. On the one hand this has for its consequence that the components of the switching arrangement must be overrrated according to the fluctuations of the power absorbed between zero value and twice the mains output power, which leads to a significantly higher costs. This overrating applies more in particular to the transformer provided in the switched-mode power supply. On the other hand, an oscillation of twice the mains frequency is superposed on the output direct voltage of the circuit arrangement of this design. Therefore, in many cases the circuit arrangement must be followed by a further stabilizing circuit which may include a linear control or a further switching regulator. This also raises the design effort and the costs of the circuit arrangement to a significant extent.
The EP-B1-0 223 315 discloses a circuit arrangement for producing a direct voltage from a sinusoidal input voltage, by which the noise voltages at lower frequencies are reduced. The known circuit arrangement includes for that purpose a switched-mode power supply comprised of a diode, a coil, a capacitor and a transistor, to which via a rectifier a basically sinusoidal input voltage is applied and whose elements are arranged such, that in the conductive state of the transistor the diode is non-conductive and the coil current flows at least via the transistor and in the non-conductive state via the diode and a parallel circuit formed by a load and the capacitor. In a pulse generator switching pulses for the transistor are produced, from the input voltage, whose frequency changes monotonously versus time between a minimum frequency when the rectified input voltage has its maximum value, and a maximum frequency at the minimum value. In this situation the load may include a (further) switched-mode power supply, via which the end user is fed. Consequently, a switched-mode power supply which does not provide an electrical insulation is connected in this circuit arrangement between the rectifier and the capacitor. This (further) switched-mode power supply is preferably constituted by an up-converter, by means of which a sinusoidal current can be taken from mains and which simultaneously produces a voltage across the capacitor which exceeds the peak of the mains a.c. voltage. Consequently, the subsequent switched-mode power supply inside the load, which likewise includes a transformer, can be rated for the means output power, as no power peaks occur any more. As this (further) switched-mode power supply is fed with a high input voltage, the currents occurring therein and consequently also the associated ohmic losses are relatively low there. However, the reduced circuit cost and design effort obtained for the (further) switched-mode power supply is offset by the additional cost and design effort for the up-converter, which as described basically comprises a coil, a free-running diode and a high frequency-operated switch with matching drive circuit.
From the DE-OS 35 37 536 or from the article "Snubber Circuits: Theory, Design and Application" by Philip C. Todd, published in "Unitrode Switching Regulated Power Supply Design Manual", May 1993, attenuators (so-called snubber or voltage clamps) are known, by means of which an attenuation, beset with losses, of flyback pulses at high-frequency switches can be effected. Circuits of this type can more specifically be used to reduce high-frequency interferences, as they limit the rise time of the cut-off frequency at the high-frequency switch. Namely, the faster the voltage at the high-frequency switch can rise on switching to the non-conductive state, the greater the capacitive noise currents flowing in the capacitances always present in the region of the high-frequency switch, for example between a terminal of the high-frequency switch and ground. If such interferences are not previously damped, then expensive mains filters may possibly be required to suppress them. More in particular, interferences of this type may occur at switched-mode power supplies in monitors or television sets, but also in other, comparably sensitive devices, as there they can, for example, become visible as interferences in the picture.
But also the attenuators described form an additional circuit design effort and cost. The design effort and cost becomes particularly high when the described, known measures to suppress the low-frequency noise and the high-frequency noise must be used in combination in a current supply circuit.